An 8×64 Gb/s PAM-4 Retimed Optical Receiver with Forwarded Clock for UCIe Compliant Optical I/O in 28-nm CMOS

This work presents an 8×64 Gb/s four-level pulse amplitude modulation (PAM-4) retimed optical receiver (ORX) with forwarded clock for universal chiplet interconnect express (UCIe) compliant optical I/O (OIO). The AFE employs a transadmittance-stage transimpedance-stage (TAS-TIS) where a dual-gain path single-to-differential (S2D) topology is proposed to enhance linearity for PAM-4 signaling. A shared forwarded optical clock (FOC) network is adopted for better jitter tracking and higher area efficiency. A two-stage high-speed slicer is proposed to realize 64 Gb/s PAM-4 signal sampling. The proposed ORX is implemented in a 28-nm CMOS process and co-packaged with a dedicated photodetector (PD) array via flip-chip bonding, enabling 512 Gb/s PAM-4 aggregated data transmission with a bandwidth density of 955 Gb/s/mm² and an energy efficiency of 2.25 pJ/bit.